Owing to the shortage of fresh water sources, sea-water or brackish water is being increasingly viewed around the world as a viable and long term source of fresh water. However, the saline nature of sea-water renders it unfit for use as fresh water. Thus different desalination techniques, such as reverse osmosis have been used to desalinate sea-water in significant quantities in order to provide pure drinking water in large. In reverse osmosis, the water to be desalinated is forced through a semi-permeable membrane so that the dissolved solids are removed by the membrane.
Reverse osmosis is similar to a filtration process in which sea-water is pumped in under high pressure through a membrane that retains the concentrated solute, such as dissolved salt ions on one side and allows the pure solvent, such as fresh water to pass to the other side. The fresh water leaves the membrane under relatively low pressure while the concentrated solvent containing the dissolved solutes leaves the membrane under high pressure, but some bit lower than the pressure of the sea-water supplied to the membrane. The concentrated solvent is then discharged into the sea, leading to wastage of the energy latent in the concentrated solvent in the form of pressure energy. Thus the desalination process has been found to be inefficient with high energy costs.
Thus, different means have been employed to recover the energy latent in the concentrated solvent. Some of the already proposed energy recovery schemes for the recovery of the otherwise lost energy use water turbines with high cost. Alternatively, many energy recovery schemes have proposed the use of reciprocating pistons for reducing the energy required to pump in the sea-water to high pressure for a reverse osmosis unit. However the use of reciprocating pistons places extra energy demands on the system. Moreover, a further disadvantage of the use of reciprocating pistons is that some energy may be lost in overcoming the friction generated during the action of the pistons.
Accordingly, there is a need for improved systems and methods for recovering energy from the concentrated solvent in order to increase the efficiency of the desalination process.